Mutant hamster cells resistant to N-phosphonacetyl-L-aspartate (PALA), a potent inhibitor of aspartate transcarbamylase, greatly over-accumulate a multifunctional protein that catalyzes the first 3 reactions of the de novo pyrimidine pathway. To seek the cause(s) of over-accumulation, we will isolate and characterize the specific mRNA for this protein, make cDNA from it, and use this cDNA probe to determine the levels of mRNA and mRNA precursors and the number of structural genes in several resistant clones. The cDNA will also be cloned, and the DNA so obtained will be coupled covalently to cellulose for isolation of mRNA precursors and mRNP particles. Hybrids derived either from resistant plus sensitive cells or from two different resistant cells will be used to evaluate dominance of the resistant phenotype and to categorize different mutants into complementation groups. With labeled cDNA and restriction fragments of hamster DNA, the chromosomal environment(s) of the gene in wild type and mutant cells will be compared. In a lytic infection by SV40, viral DNA molecules seem to be directed toward different functions (early transcription, late transcription, DNA replication, packaging) by the nature of proteins bound to a central control region. For example, T antigen probably shuts off early transcription and turns on DNA replication simultaneously when it binds to this region. We will study the participation of different viral proteins in different nucleoprotein complexes and in different phases of virus growth by using specific antibodies to visualize these proteins in the electron microscope. In addition, we will study the processing of those viral RNA's that can be produced in large amounts (late RNA late in infection or early RNA in a tsA mutant) and also the mechanism of control of late transcription, using viral mutants.